// Copyright (c) 2011 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// PLEASE READ: Do you really need a singleton?
//
// Singletons make it hard to determine the lifetime of an object, which can
// lead to buggy code and spurious crashes.
//
// Instead of adding another singleton into the mix, try to identify either:
//   a) An existing singleton that can manage your object's lifetime
//   b) Locations where you can deterministically create the object and pass
//      into other objects
//
// If you absolutely need a singleton, please keep them as trivial as possible
// and ideally a leaf dependency. Singletons get problematic when they attempt
// to do too much in their destructor or have circular dependencies.

#ifndef BASE_MEMORY_SINGLETON_H_
#define BASE_MEMORY_SINGLETON_H_

#include "base/at_exit.h"
#include "base/atomicops.h"
#include "base/base_export.h"
#include "base/macros.h"
#include "base/memory/aligned_memory.h"
#include "base/threading/thread_restrictions.h"

namespace base {
namespace internal {

    // Our AtomicWord doubles as a spinlock, where a value of
    // kBeingCreatedMarker means the spinlock is being held for creation.
    static const subtle::AtomicWord kBeingCreatedMarker = 1;

    // We pull out some of the functionality into a non-templated function, so that
    // we can implement the more complicated pieces out of line in the .cc file.
    BASE_EXPORT subtle::AtomicWord WaitForInstance(subtle::AtomicWord* instance);

    class DeleteTraceLogForTesting;

} // namespace internal

// Default traits for Singleton<Type>. Calls operator new and operator delete on
// the object. Registers automatic deletion at process exit.
// Overload if you need arguments or another memory allocation function.
template <typename Type>
struct DefaultSingletonTraits {
    // Allocates the object.
    static Type* New()
    {
        // The parenthesis is very important here; it forces POD type
        // initialization.
        return new Type();
    }

    // Destroys the object.
    static void Delete(Type* x)
    {
        delete x;
    }

    // Set to true to automatically register deletion of the object on process
    // exit. See below for the required call that makes this happen.
    static const bool kRegisterAtExit = true;

#ifndef NDEBUG
    // Set to false to disallow access on a non-joinable thread.  This is
    // different from kRegisterAtExit because StaticMemorySingletonTraits allows
    // access on non-joinable threads, and gracefully handles this.
    static const bool kAllowedToAccessOnNonjoinableThread = false;
#endif
};

// Alternate traits for use with the Singleton<Type>.  Identical to
// DefaultSingletonTraits except that the Singleton will not be cleaned up
// at exit.
template <typename Type>
struct LeakySingletonTraits : public DefaultSingletonTraits<Type> {
    static const bool kRegisterAtExit = false;
#ifndef NDEBUG
    static const bool kAllowedToAccessOnNonjoinableThread = true;
#endif
};

// Alternate traits for use with the Singleton<Type>.  Allocates memory
// for the singleton instance from a static buffer.  The singleton will
// be cleaned up at exit, but can't be revived after destruction unless
// the Resurrect() method is called.
//
// This is useful for a certain category of things, notably logging and
// tracing, where the singleton instance is of a type carefully constructed to
// be safe to access post-destruction.
// In logging and tracing you'll typically get stray calls at odd times, like
// during static destruction, thread teardown and the like, and there's a
// termination race on the heap-based singleton - e.g. if one thread calls
// get(), but then another thread initiates AtExit processing, the first thread
// may call into an object residing in unallocated memory. If the instance is
// allocated from the data segment, then this is survivable.
//
// The destructor is to deallocate system resources, in this case to unregister
// a callback the system will invoke when logging levels change. Note that
// this is also used in e.g. Chrome Frame, where you have to allow for the
// possibility of loading briefly into someone else's process space, and
// so leaking is not an option, as that would sabotage the state of your host
// process once you've unloaded.
template <typename Type>
struct StaticMemorySingletonTraits {
    // WARNING: User has to deal with get() in the singleton class
    // this is traits for returning NULL.
    static Type* New()
    {
        // Only constructs once and returns pointer; otherwise returns NULL.
        if (subtle::NoBarrier_AtomicExchange(&dead_, 1))
            return NULL;

        return new (buffer_.void_data()) Type();
    }

    static void Delete(Type* p)
    {
        if (p != NULL)
            p->Type::~Type();
    }

    static const bool kRegisterAtExit = true;
    static const bool kAllowedToAccessOnNonjoinableThread = true;

    // Exposed for unittesting.
    static void Resurrect() { subtle::NoBarrier_Store(&dead_, 0); }

private:
    static AlignedMemory<sizeof(Type), ALIGNOF(Type)> buffer_;
    // Signal the object was already deleted, so it is not revived.
    static subtle::Atomic32 dead_;
};

template <typename Type>
AlignedMemory<sizeof(Type), ALIGNOF(Type)>
    StaticMemorySingletonTraits<Type>::buffer_;
template <typename Type>
subtle::Atomic32 StaticMemorySingletonTraits<Type>::dead_ = 0;

// The Singleton<Type, Traits, DifferentiatingType> class manages a single
// instance of Type which will be created on first use and will be destroyed at
// normal process exit). The Trait::Delete function will not be called on
// abnormal process exit.
//
// DifferentiatingType is used as a key to differentiate two different
// singletons having the same memory allocation functions but serving a
// different purpose. This is mainly used for Locks serving different purposes.
//
// Example usage:
//
// In your header:
//   template <typename T> struct DefaultSingletonTraits;
//   class FooClass {
//    public:
//     static FooClass* GetInstance();  <-- See comment below on this.
//     void Bar() { ... }
//    private:
//     FooClass() { ... }
//     friend struct DefaultSingletonTraits<FooClass>;
//
//     DISALLOW_COPY_AND_ASSIGN(FooClass);
//   };
//
// In your source file:
//  #include "base/memory/singleton.h"
//  FooClass* FooClass::GetInstance() {
//    return Singleton<FooClass>::get();
//  }
//
// And to call methods on FooClass:
//   FooClass::GetInstance()->Bar();
//
// NOTE: The method accessing Singleton<T>::get() has to be named as GetInstance
// and it is important that FooClass::GetInstance() is not inlined in the
// header. This makes sure that when source files from multiple targets include
// this header they don't end up with different copies of the inlined code
// creating multiple copies of the singleton.
//
// Singleton<> has no non-static members and doesn't need to actually be
// instantiated.
//
// This class is itself thread-safe. The underlying Type must of course be
// thread-safe if you want to use it concurrently. Two parameters may be tuned
// depending on the user's requirements.
//
// Glossary:
//   RAE = kRegisterAtExit
//
// On every platform, if Traits::RAE is true, the singleton will be destroyed at
// process exit. More precisely it uses AtExitManager which requires an
// object of this type to be instantiated. AtExitManager mimics the semantics
// of atexit() such as LIFO order but under Windows is safer to call. For more
// information see at_exit.h.
//
// If Traits::RAE is false, the singleton will not be freed at process exit,
// thus the singleton will be leaked if it is ever accessed. Traits::RAE
// shouldn't be false unless absolutely necessary. Remember that the heap where
// the object is allocated may be destroyed by the CRT anyway.
//
// Caveats:
// (a) Every call to get(), operator->() and operator*() incurs some overhead
//     (16ns on my P4/2.8GHz) to check whether the object has already been
//     initialized.  You may wish to cache the result of get(); it will not
//     change.
//
// (b) Your factory function must never throw an exception. This class is not
//     exception-safe.
//

template <typename Type,
    typename Traits = DefaultSingletonTraits<Type>,
    typename DifferentiatingType = Type>
class Singleton {
private:
    // Classes using the Singleton<T> pattern should declare a GetInstance()
    // method and call Singleton::get() from within that.
    friend Type* Type::GetInstance();

    // Allow TraceLog tests to test tracing after OnExit.
    friend class internal::DeleteTraceLogForTesting;

    // This class is safe to be constructed and copy-constructed since it has no
    // member.

    // Return a pointer to the one true instance of the class.
    static Type* get()
    {
#ifndef NDEBUG
        // Avoid making TLS lookup on release builds.
        if (!Traits::kAllowedToAccessOnNonjoinableThread)
            ThreadRestrictions::AssertSingletonAllowed();
#endif

        // The load has acquire memory ordering as the thread which reads the
        // instance_ pointer must acquire visibility over the singleton data.
        subtle::AtomicWord value = subtle::Acquire_Load(&instance_);
        if (value != 0 && value != internal::kBeingCreatedMarker) {
            return reinterpret_cast<Type*>(value);
        }

        // Object isn't created yet, maybe we will get to create it, let's try...
        if (subtle::Acquire_CompareAndSwap(&instance_, 0,
                internal::kBeingCreatedMarker)
            == 0) {
            // instance_ was NULL and is now kBeingCreatedMarker.  Only one thread
            // will ever get here.  Threads might be spinning on us, and they will
            // stop right after we do this store.
            Type* newval = Traits::New();

            // Releases the visibility over instance_ to the readers.
            subtle::Release_Store(&instance_,
                reinterpret_cast<subtle::AtomicWord>(newval));

            if (newval != NULL && Traits::kRegisterAtExit)
                AtExitManager::RegisterCallback(OnExit, NULL);

            return newval;
        }

        // We hit a race. Wait for the other thread to complete it.
        value = internal::WaitForInstance(&instance_);

        return reinterpret_cast<Type*>(value);
    }

    // Adapter function for use with AtExit().  This should be called single
    // threaded, so don't use atomic operations.
    // Calling OnExit while singleton is in use by other threads is a mistake.
    static void OnExit(void* /*unused*/)
    {
        // AtExit should only ever be register after the singleton instance was
        // created.  We should only ever get here with a valid instance_ pointer.
        Traits::Delete(reinterpret_cast<Type*>(subtle::NoBarrier_Load(&instance_)));
        instance_ = 0;
    }
    static subtle::AtomicWord instance_;
};

template <typename Type, typename Traits, typename DifferentiatingType>
subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>::instance_ = 0;

} // namespace base

#endif // BASE_MEMORY_SINGLETON_H_
